Other musicians might prefer a piezo pickup or “contact mic,” which transduces the vibrations of the instrument body directly into sound. A similar approach is a magnetic pickup, which is sensitive only to string vibrations. This direction can deliver more gain before feedback than a mic as well as much-improved isolation, but it can also result in an “electric” rather than “acoustic” sound, missing the resonance of the instrument’s body and air chamber.
Some EQ can help – try a narrow cut at 1.2 to 1.5 kHz, along with some high-frequency roll-off. Pickups also prevent phase cancellations between two mics deployed for a singing guitarist.
Hybrid systems combine a pickup with a contact mic, providing a blend of high volume from the pickup and “air” from the mic. Usually, the performer decides on the blend and polarity of the two sources. Here’s a trick to prevent feedback with this configuration: Send just the pickup signal to the stage monitors and send just the mic signal to the house loudspeakers.
In the mixer channel for the pickup, turn down the fader and turn up the monitor send. In the mixer channel for the mic, turn up the fader and turn down the monitor send. The monitors don’t feed back and the audience hears the true timbre of the instrument.
Because a pickup has high output impedance, it needs to be loaded by a high-Z input, ideally about 1 megohms. Many active direct boxes can provide that input impedance, but most passive (transformer-coupled) DIs do not. They load down the pickup and result in a “nasally” or “honky” sound. For this reason, many performers feed their pickup into a preamp with a high-Z input and low-Z output.
Many engineers have also had success with a hybrid method that combines a pickup with a mini mic. A pickup mounted under the bridge picks up the lows and provides volume and punch. A mini hypercardioid mic is mounted just inside the sound hole facing in. It provides the treble and clean acoustic string sound.
The pickup and mic are mixed in a small 2-input mixer provided as part of the system. The combination of the pickup and microphone provides a loud, punchy, yet natural sound with all the crispness of a real acoustic guitar.
Here are some additional techniques in “cheat sheet” form for miking various acoustic instruments with stand-mounted mics as well as instrument-mounted miniatures. As always, there’s not a specific right way to handle this job – feel free to experiment and just do whatever works. That said…
Acoustic guitar: Place a mic about 3 or 4 inches away, just to the right of the sound hole as viewed from the audience. A mic directly in front of the sound hole makes a boomy effect because the sound hole resonates around 80 to 100 Hz. If the amplified sound has too much bass or is too “thumpy,” roll off some lows. If the sound is too harsh, cut around 3 kHz.
Violin: A mic about 8 inches over the bridge works well. Aim it toward the f-holes for a warmer sound, or toward the neck for a thinner sound. A singing fiddler can be miked with a single microphone aiming at the player’s chin.
Banjo: Aim at a spot halfway between the bridge and the lower edge, about 3 inches away. If the sound is too hollow, the player can stuff a rag inside the banjo. You might also cut a little around 500 Hz.
Mandolin: Many mandos have a thin, harsh sound. You can warm it up by close-miking the lower f-hole. Adjust EQ to taste.
Upright bass: An EQ’d pickup can be the best choice to prevent leakage and feedback. When miking, avoid placing it directly over the f-hole because the sound there is muddy and hollow. A mic located a few inches under the bridge, aimed at the body, can capture a deep, tight sound. You might mix in another mic close to the plucking fingers for definition and roll off the lows in the pluck mic.
It’s common to wrap a Shure SM57 (or similar) in foam behind the tailpiece, with the mic aiming up toward the bridge. This lets the bassist move around more freely. Make sure that the front grille is not covered in foam and apply EQ to get a natural sound.
Hammered dulcimer: Place a mic about 8 inches over the front edge aiming at the center of the soundboard.
Lap dulcimer: Aim a mic down at a soundhole a few inches away.
Dobro: This instrument is basically a guitar held on the lap and played slide-style with a bottleneck. You can mic the soundhole a few inches away and roll off the lows if the sound is too bassy. Roll off some highs if the sound is too cluttered and bright.
Amplifying acoustic music presents a different set of approaches and circumstances than the typical amplified rock show. But if you can capture the delicate sound of acoustic instruments, your audience is in for a beautiful sonic treat.